U.S. patent number 4,833,971 [Application Number 07/165,762] was granted by the patent office on 1989-05-30 for self-regulated hydraulic control system.
Invention is credited to Philip A. Kubik.
United States Patent |
4,833,971 |
Kubik |
May 30, 1989 |
Self-regulated hydraulic control system
Abstract
A self-regulating hydraulic control circuit for controlling the
flow of fluid to and from the cylinder of a double-acting hydraulic
motor employs two pumps, one of which supplies fluid at a
relatively high pressure and low displacement while the second pump
is a relatively high displacement pump which operates only against
a relatively low pressure requirement. The system is especially
adapted for applications where, during a working stroke, the piston
of the hydraulic motor moves from a start position through a first
portion of the stroke during which the piston encounters but a
minimal resistance to movement and then suddenly encounters a high
resistance to movement. One example of such an application would be
a press in which, at some point during the stroke of the piston, a
tool carried by the piston initially encountered a workpiece. The
control circuit is so designed that during the initial or free
travel portion of the piston stroke, a low volume-high pressure
pump is connected in a closed loop system via a directional control
valve to two opposed faces of the piston which are of equal area.
The high displacement pump is connected to act against a third face
of the piston. The high volume of fluid supplied from the high
volume pump will drive the piston rapidly through the free travel
portion of its stroke. The increased pressure in the fluid circuit
resulting from contact between a piston carried tool and a
workpiece is employed to disconnect the high volume pump from the
cylinder to avoid overloading the drive motor of that pump at the
same time the high pressure pump is connected to apply pressure
against the third face of the piston.
Inventors: |
Kubik; Philip A. (Bloomfield
Hills, MI) |
Family
ID: |
22600353 |
Appl.
No.: |
07/165,762 |
Filed: |
March 9, 1988 |
Current U.S.
Class: |
91/29; 60/430;
91/512; 60/428; 91/511; 91/519 |
Current CPC
Class: |
B30B
15/161 (20130101); F15B 11/036 (20130101); F15B
15/1466 (20130101); F15B 2211/775 (20130101); F15B
2211/20515 (20130101); F15B 2211/20538 (20130101); F15B
2211/20546 (20130101); F15B 2211/20584 (20130101); F15B
2211/30565 (20130101); F15B 2211/31535 (20130101); F15B
2211/329 (20130101); F15B 2211/355 (20130101); F15B
2211/50518 (20130101); F15B 2211/55 (20130101); F15B
2211/7055 (20130101) |
Current International
Class: |
B30B
15/16 (20060101); F01B 029/04 (); F16D
031/02 () |
Field of
Search: |
;91/28,29,32,33,511,512,519 ;60/493,428,430 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Look; Edward K.
Assistant Examiner: Denion; Thomas
Attorney, Agent or Firm: Basile and Hanlon
Claims
What is claimed is:
1. In a hydraulic system including main fluid pump means having an
intake and an outlet for pumping a relatively low volume of fluid
at a relatively high pressure from said outlet, a reciprocatory
fluid motor including a cylinder having a head end port and a rod
end port and a piston including a rod projecting from the rod end
of said cylinder, and directional control valve means for
operatively connecting the intake and outlet of said main pump
means to said rod end and head end ports in a closed loop system to
selectively extend or retract the piston rod relative to said
cylinder;
the improvement comprising second fluid pump means having an intake
connected to a sump and an outlet for discharging a relatively high
volume of fluid at a relatively low pressure from its outlet,
second valve means operable in a first position to connect the
outlet of said second pump means to the head end of said cylinder
and operable in a second position to connect the outlet of said
main pump means to the head end of said cylinder, first pilot means
for urging said second valve means toward said first position in
response to pressure applied to said first pilot means, second
pilot means for urging said second valve means toward said second
position in response to pressure applied to said second pilot
means, said first and second pilot means acting in opposition to
each other such that movement of said second valve means can occur
only when the pressure applied to one pilot means exceeds the
pressure applied to the other, first conduit means for placing said
first pilot means in fluid communication with said rod end port,
second conduit means placing said second pilot means in
communication with said head end port, said first conduit means
including a one-way check valve accommodating flow of fluid only
toward said first pilot means, normally closed valve means
connected in said first conduit means in parallel with said one-way
check valve for accommodating flow of fluid away from said first
pilot means to relieve the pressure applied to said first pilot
means when the pressure in said first pilot means exceeds a
preselected relatively high pressure, said normally closed valve
means including third pilot means for opening said normally closed
valve means when the pressure applied to said first pilot means
exceeds said preselected relatively high pressure, and fourth pilot
means operable when said normally closed valve means is open for
maintaining said normally closed valve means open independetly of
said first pilot means.
2. The invention defined in claim 1 further comprising pressure
relief means operatively connected to the outlet of said second
pump means for venting said outlet to said sump when the pressure
at said outlet exceeds a preselected relatively low pressure.
3. In a "high-low" hydraulic circuit for rapidly advancing the
piston of a hydraulic motor means from a rest position during a
first portion of a working stroke until an increased resistance to
movement of the piston is encountered and subsequently driving the
piston at a lower speed against the increased resistance to
complete said working stroke, said system including a first
relatively low-displacement, high-pressure pump having an intake
and an outlet, a second relatively high-displacement, low-pressure
pump having an intake and an outlet, and hydraulic circuit means
for connecting said first and second pump means to said fluid motor
means to drive said piston through said working stroke and to
subsequently rapidly return said piston to said rest position;
the improvement wherein said hydraulic motor means includes a
cylinder having a piston operatively mounted therein for
reciprocatory movement and having a rod projecting from a rod end
of the cylinder, means on said cylinder and piston defining first,
second and third working faces on said piston respectively exposed
to first, second and third hydraulically separate chambers within
said cylinder, said first and second faces being of equal area and
facing opposite ends of said cylinder and said third face facing in
the same direction as said first face and having an area greater
than that of said first face, first, second and third ports in said
cylinder communicating respectively with said first, second and
third chambers;
and said hydraulic circuit means comprises a multi-position
reversing valve controlled first circuit means for connecting the
intake and outlet of said first pump to said first and second ports
of said motor means in a closed-loop circuit to selectively enable
said first pump to drive said piston in a working stroke from said
rest position or a return stroke to said rest position in
accordance with the position of said reversing valve, second
circuit means including a pilot actuated, two-position valve
operable in a first position to connect said third port to the
outlet of said first pump and operable in a second position to
connect said third port to the outlet of said second pump, said
two-position valve including opposed first and second pressure
actuated pilots operable to position said two-position valve in its
first position when the pressure in said first pilot exceeds the
pressure in said second pilot and to position said two-position
valve in said second position when the pressure in said second
pilot exceeds the pressure in said first pilot, first conduit means
in said second circuit means connecting said second pilot to said
first port, second conduit means in said second circuit means
connecting said first pilot to the intake side of one of said
pumps, a dual-stage, pilot-actuated sequence valve in said second
conduit means operable to maintain a normally closed position
blocking flow from said first pilot until the pressure in said
first pilot rises above a selected first pressure at which said
sequence valve opens and to remain open until the pressure in said
first pilot drops below a second pressure substantially less than
said first pressure, and a one-way check valve connected in said
second conduit means in parallel with said sequence valve
accommodating flow toward said first pilot.
4. The invention defined in claim 3 further comprising pressure
relief valve means connected to the outlet of said second pump, and
replenishing means for connecting the outlet of said second pump to
the intake of said first pump.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a self-regulating hydraulic
control system employed to drive the piston of an hydraulic
cylinder in a working cycle in which, during its working stroke,
the piston moves against a minimal resistance during an initial or
"free travel" portion of its extending stroke and then encounters a
substantial resistance to further movement as, for example, in a
punch press when the punch driven by the piston initially
encounters the workpiece.
In such applications, of which a punch press is but one example, to
minimize cycle time it is desirable to supply a high volume flow of
fluid, which may be at a relatively low pressure, to the head end
of the cylinder to drive the piston in rapid movement through the
free travel portion of its working stroke to rapidly bring the tool
or punch into contact with the work. Once the punch is engaged with
the work, however, a substanial increase in pressure applied to the
head end of the cylinder will be required to drive the punch
through the work. This phase of the working stroke may be most
efficiently accomplished, from the standpoint of the power
requirement, by a pump which discharges a relatively low volume of
fluid at a relatively high pressure. Where a single pump is
employed to drive a piston in such a working stroke, the tonnage
required during the working portion of the stroke will impose a
compromise between minimizing cycle time and minimizing the
horsepower requirements for the motor which drives the pump.
2. Description of the Prior Art
This problem has been recognized in the prior art and has resulted
in various so called "high-low" systems which, by employing two
pumps acting in conjunction with each other during the free travel
portion of the piston stroke, achieve relatively rapid cycle times
with a power requirement which is but a fraction of the power
requirement of a single pump system. In such systems, the main
pump, which will power the cylinder during the working portion of
its stroke, is a relative low volume pump which can produce the
desired tonnage with a relatively low power input requirement. The
second pump is a relatively high volume pump which, when driven by
the same motor which drives the main pump, will put out a
relatively high volume of fluid at a relatively low pressure.
Typically, the system is so designed that both pumps supply fluid
to the cylinder during the free travel portion of the piston stroke
and the output of the high volume pump is diverted from the
cylinder during the working portion of the piston stroke. Such
systems typically disconnect the output of the high volume pump
from the working circuit by mechanically or electrically actuated
valves which, in turn, rely upon pressure switches, limit switches,
cams, etc., to control their actuation. These systems, in general,
have proven difficult to maintain and troubleshooting is frequently
complicated by the initial problem of determining whether the fault
lies with the mechanical, electrical or hydraulic components of the
system.
The present invention is directed to a two-pump system in which the
system is self-regulating in response to its own internal operating
pressures.
SUMMARY OF THE INVENTION
A system embodying the present invention employs a fluid pressure
actuated single rod piston-cylinder assembly. Such assemblies are
known in the art, see, for example, U.S. Pat. No. 3,744,375. The
cylinder and piston are so constructed as to provide three separate
fluid chambers within the cylinder with three separate faces on the
piston respectively exposed to the three chambers. Fluid contained
in a first and a second of the chambers works against respective
first and second faces of the piston, the first face being a head
end face and the second face being a rod end face. These first and
second faces are of equal area and, thus, may be hydraulically
connected in a closed loop system via a conventional four-way valve
to the intake and discharge outlet of a fluid pump. In the
disclosed system, the third face of the piston is a head end face
and the third chamber of the piston may be selectively connected to
the discharge outlet of either of a high volume-low pressure pump
or a low volume-high pressure pump. The low volume-high pressure
pump, during operation, is connected via the aforementioned
four-way valve to the first and second chambers throughout the
entire range of movement of the piston.
During the initial free travel portion of the extending stroke of
the piston, the outlet of the high volume-low pressure pump is
connected to the third chamber via a two-position dual pilot
operated valve. During this portion of the stroke, the piston
encounters but a minimal resistance to movement and the high volume
of fluid supplied to the cylinder by the high volume pump drives
the piston at a relatively rapid rate of movement during the free
travel portion of the extending piston stroke. When the piston
encounters a substantially increased resistance to extending
movement, as when a tool carried by the piston engages a stationary
workpiece, this increased resistance generates a dramatically
increased pressure in the first chamber of the cylinder. Pressure
in the first chamber of the cylinder is applied to one pilot of the
dual position valve. The opposite pilot of the two-position valve
is connected via a fluid circuit to communicate with the second
chamber of the piston. This latter fluid circuit includes a one-way
check valve oriented to accommodate flow in a direction from the
second chamber toward the opposed pilot and to block flow of fluid
from the second pilot to the second chamber. The pressure applied
to the first pilot is opposed by whatever pressure exists in the
second pilot. To release pressure from the second pilot, the fluid
circuit includes a dual stage sequence valve connected in parallel
bypassing relationship to the one-way check valve. The dual stage
valve is operable to remain in a closed position until pressure
within the second pilot rises above a preselected relatively high
pressure. Once the dual stage valve is opened by this relatively
high pressure, it will be maintained open until the pressure in the
second pilot falls to a pressure substantially below the pressure
required to initially open the valve. With the foregoing
arrangement, the initial opening pressure of the dual stage valve
is set to substantially exceed the pressure in the second chamber
of the cylinder during the free travel portion of the piston
stroke. When the piston encounters a relatively high resistance to
further movement, as by the engagement of a tool with a workpiece,
the increased pressure in the second chamber rises to a pressure
greater than that required to open the dual stage valve, and
opening of the dual stage valve shifts the valve to disconnect the
high volume pump from the cylinder and to connect the output of the
low volume-high pressure pump to the third chamber. Thus, when the
piston encounters a high resistance to further extending movement,
the output of the high pressure pump is applied to the third
chamber of the cylinder to substantially increase the force exerted
by the piston.
Other objects and features of the invention will become apparent by
reference to the following specification and to the drawings.
IN THE DRAWINGS
The single FIGURE of drawings shows a schematic diagram of a
hydraulic control system embodying the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The system shown in the drawings includes a main pump, designated
generally 10, and a second pump, designated generally 12, both of
which are driven by a power source, such as an electric motor,
designated generally 14. Pumps 10 and 12 effectively operate as
constant displacement pumps whose power requirements are directly
proportional to the pressure requirements of the system to be
driven by the pump. However, in many applications, main pump 10
preferably is a variable displacement pump provided with a control
which will vary its displacement to provide appropriate
acceleration and deceleration of the driven piston at the start and
end of the piston stroke to prevent a shock loading of the
hydraulic circuit. Except for these relatively short periods of
increasing and decreasing displacement at the start and finish of
each cycle, the pump operates at a constant maximum
displacement.
In the present system, pumps 10 and 12 are employed to drive the
piston 16 of a single rod hydraulic cylinder 18 in an application
where the piston rod 20, when driven in a working stroke in which
the rod is extended from cylinder 18, will move against a minimal
resistance during the initial portion of the stroke and, at some
time before the completion of the extending stroke, will encounter
a high resistance to further extending movement. A typical example
of such an application is a punch press in which a punch carried by
the piston rod will be driven from a retracted position into
contact with a workpiece and then driven through the workpiece. In
order to drive the punch through the workpiece, the system pump
must be capable of supplying fluid at a relatively high pressure,
of the order of 2,000 PSI, for example, while pressures of this
magnitude are obviously not required to drive the piston during the
free travel portion of its stroke before the punch engages the
workpiece. Because the horsepower requirement of the motor which
drives the pump is directly proportional to the product of the pump
displacement and pump output pressure, where a high output pressure
is required, cost considerations normally will dictate the use of a
relatively low displacement pump to avoid an unreasonably high
horsepower requirement for the motor employed to drive the pump.
Conversely, in the punch press example referred to above, during
the free travel portion of the working stroke, a relatively low
pressure, of the order of 150 PSI, for example, will be adequate to
drive the piston. During this portion of the piston stroke, a
relatively large displacement pump could be employed without
imposing an unreasonable power requirement upon the pump drive
motor. To minimize cycle time, it would be desirable to drive the
piston relatively rapidly through the free travel portion of its
stroke.
Accordingly, in the system shown in the drawings, pump 10 is
intended to be capable of applying the maximum pressure required by
the piston and, thus, to achieve a reasonably low power
requirement, will be a relatively low displacement pump. Pump 12,
on the other hand, is intended to supply a relatively high volume
of fluid at a relatively low pressure. The two pumps work in
conjunction during the free travel portion of the stroke, while
only pump 10 supplies pressure to the piston during the working
portion of the stroke. The system is so designed as to
automatically regulate itself to achieve the foregoing result.
As shown in the drawings, the pressure or outlet side of main pump
10 is connected by a pressure conduit 10P to the pressure port 22
of a three-position four-way valve, designated generally 24, while
the return port 26 of valve 24 is connected via a return conduit
10R to the intake of pump 10. With valve 24 centered, as shown in
the drawings, an internal passage 28 in valve 24 connects ports 22
and 26 to each other so that pump 10 may idle between working
cycles. At the opposite side of valve 24, control port 29 of the
valve is connected via a head end conduit 30 to a head end port 31
of cylinder 18 while the second control port 32 of valve 24 is
connected via rod end conduit 34 to a rod end port 35 of cylinder
18. With the valve 24 in the centered position shown, control ports
29 and 32 are blocked within valve 24.
Valve 24 is normally spring centered and is provided in a well
known manner with actuators 36, 38 which are operable to shift the
valve from its centered position to either of its two operating
positions to drive piston 16 in an extending stroke when the
straight-through connections of valve 24 are aligned with the ports
or to retract piston 16 when the cross connections of valve 24 are
aligned with the valve port. Actuators 36, 38 may take any of
several conventional forms, the particular application of the
system generally determining whether solenoid actuated, pilot
operated, or mechanical shifting would be preferable. It is
believed apparent that pump 10, valve 24 and the conduits
connecting pump 10 to cylinder 18 via valve 24 constitute a closed
loop system.
The hydraulic motor constituted by the piston-cylinder assembly
16-18 is shown and described in detail in my earlier U.S. Pat. No.
3,744,375. The piston is formed with three working faces 16A, 16B,
16C, each exposed to hydraulically separate chambers within
cylinder 18. Piston faces 16A and 16B are of equal area and are on
hydraulically opposite sides of the piston. This arrangement
enables the faces 16A and 16B to be incorporated in a closed loop
system. Face 16C serves a dual purpose function during the working
(extending) stroke of piston 16 which will be described in greater
detail below.
One function of second pump 12 in the system shown is to operate as
a charge pump which functions to charge and supply additional
fluid, as required, to the closed loop system constituted by pump
10, valve 24 and cylinder 18. The intake of pump 12 is connected to
a sump or tank 40 which maintains a supply of fluid at all times.
The pressure side or outlet of pump 12 is connected to a conduit
12P which, in turn, is connected via a branch conduit 42 to supply
fluid to the main pump circuit as required via one-way check valves
44, 46. Outlet conduit 12P also feeds a second branch conduit 48
which is connected via a conduit 50 to a relief valve 52 whose
outlet is connected to the sump or tank 40. Valve 52 is normally
spring biased to a closed position and is set to open when the
pressure at the outlet side of pump 12 exceeds a predetermined
pressure. Typically, in the system shown, valve 52 will be set to
open at 150 PSI and, in the circuit shown in the drawings, this
means that a minimum pressure of 150 PSI will be maintained at the
intake side (conduit 10R) of main pump 10.
The circuit shown also includes a two-position double-pilot
operated valve, designated generally 54. At the lower side of valve
54, as shown in the drawings, a first port 56 is connected to the
pressure conduit 12P of pump 12 by conduit 48, while a second port
58 of valve 54 is connected by a conduit 60 to the pressure conduit
10P of pump 10. At the opposite side of valve 54, one port 62 is
plugged, while the other port 64 is connected via a flow conduit 66
to a port 70 at the head end of cylinder 18.
Referring now to cylinder 18, it is seen that the cylinder includes
a hollow open-ended tube 72 projecting axially into the interior of
cylinder 18 from the head end of the cylinder. Tube 72 is slidably
and sealingly received within a blind bore 74 which extends axially
from the head end face of piston 16 into the piston rod to
terminate at a blind end 76 which constitutes face 16A of the
piston. The area of blind end 76 is equal to that of the rod end
face of the piston. The left hand end of tube 72, as viewed in the
drawings, is sealed to the head end of the cylinder, and port 70
opens through the head end of the cylinder into the interior of
tube 72.
Returning now to valve 54, opposed pressure responsive pilots 78,
80 are employed to shift the valve between its two positions. The
valve is normally spring biased to the position shown in the
drawings in which the output of pump 12 passes through the valve
from port 56 to port 64 and thence via port 70 to act against face
16C of piston 16. As stated above, valve 52 constantly maintains
the output pressure of pump 12 at the relief setting of valve 52,
for example, 150 PSI, thus with the connections as shown in the
drawings, a pressure of 150 PSI would be exerted against face 16C
of the piston, urging the piston to move to the right as viewed in
the drawings. However, such movement of piston 16 is not permitted
at this time because rod end conduit 34 is blocked at four-way
valve 24 so that fluid within cylinder 18 acting against face 16B
of the piston cannot be expelled via rod end conduit 34 to
accomodate the rightward movement of the piston.
Pilot 78 of valve 54 is connected via a conduit 82 to head end
conduit 30 of cylinder 18, while the opposite pilot 80 is connected
via a conduit 83, control circuit 84 and conduit 86 to rod end
conduit 34 of cylinder 18. Pilots 78 and 80 act in well a known
manner to position valve 54. The pilots act in direct opposition to
each other. If, for example, pressure is applied to pilot 78 to
shift the valve to the left from the position shown, shifting
movement of the valve can occur only if fluid is permitted to flow
away from the opposite pilot 80. In the configuration shown in the
drawing, flow from pilot 80 away from the pilot through conduit 83
is blocked within circuit 84 at a one-way check valve 88 and will
also be blocked at a normally closed dual-stage pilot operated
sequence valve 90 connected in circuit 84 in parallel bypassing
relationship to check valve 88.
Dual-stage valve 90 is a commercially available valve which can be
set to open in response to the application of a selected relatively
high pressure to pilot 92, but, once open, will remain open as long
as the pressure in pilot 80 exceeds a selected, substantially lower
pressure, which is applied to pilot 94.
OPERATION
Prior to the commencement of a working cycle, piston 16 will be at
its left hand end limit of movement-that is, with its piston rod
retracted into cylinder 18, as shown in the drawings. Motor 14 will
be driving both main pump 10 and the drawnings. Motor 14 will be
driving both main pump 10 and the second pump 12 with the main
directional control valve 24 in the centered position shown in the
drawing. Pilot operated valve 54 and valve 90 will likewise be in
the position shown in the drawings. With the various valves in the
positions described above, main pump 10 will idle to continuously
recirculate fluid from its outlet side through pressure conduit 10P
to port 22 of valve 24, thence through the centered valve to port
26 and back to the intake of pump 10 via return conduit 10R.
At this time, the output of pump 12 passes through conduit 12P and
conduit 48 to port 56 of valve 54. At this time, port 56 of valve
54 is connected to port 64 so that the output of pump 12 flows
through valve 54 and conduit 66 through port 70 of cylinder 18 to
act against face 16C of the piston. This pressure applied against
face 16C urges piston 16 to move to the right from the position
shown in the drawings. However, this movement cannot occur because
piston 16 is locked in the position shown due to the fact that its
rod end 34 and head end conduits 30 are blocked at directional
control valve 24 so that no displacement of piston 16 can occur.
The pressure developed by the output of pump 12 against face 16C of
the piston will back up through conduit 66, valve 54 and conduits
48, 50 to open valve 52 to connect the output of pump 12 to its
sump 40 to recirculate fluid through the circuit 12P, 48, 50, 52,
sump 40 and the intake of pump 12, while maintaining a pressure in
conduit 12P determined by the setting of valve 52. This pressure
may typically be selected as 150 PSI.
For purposes of explanation, it will be assumed that during a
working stroke of piston 16, the piston will be extended from the
cylinder to move a tool driven by the piston into engagement with a
workpiece and then drive the tool in a punching stroke through the
workpiece. During the initial portion of the stroke, the piston
encounters minimal resistance to its movement until the tool
engages the workpiece. When the tool engages the workpiece, the
resistance to further movement of the piston drastically increases.
In the system shown, main pump 12 is relied upon to supply
sufficient pressure to cylinder 18 to drive the tool through the
workpiece. The pressure required for this purpose will depend upon
many factors, such as piston area, the material of the workpiece,
etc., but will, in any event, obviously be much greater than the
pressure required to drive the piston during the initial "free
travel" portion of its stroke before the tool engages the
workpiece. If, for example, the pressure required to drive the tool
through the workpiece is 2,000 PSI, standard design tables show
that 2,000 PSI can be developed by a pump having a displacement of
3.5 gallons per minute when the pump is driven by a 4.8 horsepower
motor.
In a typical press, the application of high pressure is required
only during the final portion of the working stroke, and typically,
only over a relatively small portion of the total stroke.
The pump displacement of a constant displacement pump which can
supply the maximum required pressure when driven by a motor of
reasonably limited horsepower cannot drive the piston any faster
during the free travel portion of the piston stroke than it can
during the working portion of the stroke, and hence may have an
undesirably high cycle time.
In the present system, the second pump 12 takes the form of a
relatively high volume displacement pump which, because it is not
required to supply a relatively high pressure (maximum of 200 PSI,
for example) has a reasonably low horsepower requirement. A pump
which can supply 18 gallons per minute at 200 PSI, requires a drive
motor of only approximately 2.5 horsepower. As noted above, in the
present system, the output of pump 12 has a pressure relief valve
52 which opens at 150 PSI.
To commence a working cycle, directional control valve 24 of the
system is actuated to shift that valve from the centered position
shown in the drawing to a position where the straight through
connections of the valve are aligned with the ports to connect port
22 to port 29 and to connect ports 26 and 32 to each other. This
actuation, as described above, may be by any of several well known
and conventional control arrangements chosen in accordance with the
particular application of the system.
With the straight through connections of valve 24 aligned with the
valve ports, the pressure conduit 10P of pump 10 is connected
through valve 24 to head end conduit of cylinder 18, while the rod
end conduit 34 of cylinder 18 is connected through valve 24 to the
intake of pump 10 via return conduit 10R. With these connections
established, piston 16 will be driven to the right from the
position shown in the drawing. During this initial portion of the
"free travel" of piston 16, only a relatively small pressure is
required to move the piston.
At this time, pilot operated valve 54 is in the position shown in
the drawing and the output of pump 12 acts via port 70 to apply
pressure against face 16C of piston 16 to assist in driving the
piston to the right. As stated above, pump 12 is chosen as a high
displacement pump having an output, for example, of approximately
18 gallons per minute, and this relatively high flow rate
accommodates relatively rapid movement of piston 16 to the right
through the free travel portion of its stroke. During the free
travel portion of the stroke of piston 16, pressure in head end
conduit 30 is applied to pilot 78 of the pilot actuated valve 54
tending to shift the valve from the position shown in the drawings.
However, the opposing pilot 80 is blocked at this time by one-way
check valve 88 and the closed dual-stage valve 90. Essentially, the
pressure in line 82 applied to pilot 78 is transmitted via the
movable valve member to the opposing pilot 80 and the pressure in
line 83 leading from pilot 80 is the same as the pressure in line
82 and head end conduit 30. With the pressure thus equalized, the
spring loading of valve 54 maintains the valve in the position
shown. The setting of pilot 92 of dual-stage valve 90 is
substantially higher than the pressure which exists in line 83 at
this time and, thus, valve 90 remains closed.
At the conclusion of the free travel portion of the stroke of
piston 16, its tool engages the work and the increase of resistance
to movement of the piston to the right results in a consequent
build-up in the pressure in head end conduit 30. The same increased
pressure is applied via conduit 82 to pilot 78, which, in turn,
increases the pressure within pilot 80 until the pressure in
conduit 83 overcomes the setting of pilot 92 to enable the pilot to
shift valve 90 to its conducting position. Once valve 90 has been
so opened, its second pilot 94 will retain valve 90 in its open
position even if the pressure in line 83 drops substantially below
the setting of pilot 92. A pressure drop normally will be
encountered in a punching operation once the punch has started
cutting through the workpiece. Pressure in conduit 83 is thus
vented through valve 90 to the return side of pump 10 via conduit
86, valve 24 and return conduit 10R.
The dumping of pressure from pilot 80 permits pilot 78 to shift
valve 54 from the position shown in the drawings to establish a
connection between ports 58 and 64 of valve 54 while connecting the
output of pump 12 at port 56 to the blocked port 62 of valve 54.
This effectively disconnects pump 12 from cylinder 18 and connects
the high-pressure low-displacement pump 10 to port 70 so that the
high pressure output of pump 10 is now applied both to face 16A of
piston 16 and to face 16C. Maximum pressure is thus applied to both
head end faces of the piston, although the speed of movement of the
piston is now substantially reduced because of the relatively low
displacement of pump 10.
With the output of pump 12 plugged at port 62 of the valve 54,
pressure relief valve 52 is opened to return the output of pump 12
to its sump.
At the conclusion of the working stroke of the piston, valve 24 is
shifted by actuation of actuator 38 to place the cross connections
of the valve in alignment with its ports. The output of main pump
10 now flows from the pump through conduit 10P, ports 22 and 32 of
valve 24 and rod end conduit 34 to the rod end of cylinder 18,
while the head end of cylinder 18 is connected via conduit 30,
ports 29 and 26 and return conduit 10R to the intake of pump 10 to
drive the piston in retracting movement -- i.e., to the left as
viewed in the drawing. Rod end conduit 34 is now the high pressure
side of the circuit, and the pressure in rod end conduit 34 is
applied via conduit 86, one-way check valve 88 and conduit 83 to
pilot 80 of valve 54. The opposing pilot 78 is connected via line
82 to head end conduit 30 which is now the low pressure side of the
circuit of pump 10. The pressure applied to pilot 80 from rod end
conduit 34 is thus operable at this time to shift valve 54 back to
the position shown in the drawings, since fluid can flow directly
from pilot 78 to the low pressure side of pump 10.
Fluid exposed to face 16A of the piston can now discharge through
ports 70, conduit 66, ports 64 and 56 and, thence, via conduits 48,
50 and valve 52 to the intake side or sump 40 of pump 12. Although
pump 12, at this time, is attempting to pump fluid to port 70 of
cylinder 18, the pressure applied to face 16B of the piston 16 by
main pump 10 is much higher and the setting of valve 52 is overcome
easily to accommodate the discharge of fluid from port 70 into the
sump of pump 12. The relatively small area of face 16B of piston 16
enables a reasonably rapid rate of movement of the piston 16 during
its retracting stroke.
While one embodiment of the invention has been described in detail,
it will be apparent to those skilled in the art that the disclosed
embodiment may be modified. Therefore, the foregoing description is
to be considered exemplary, rather than limiting, and the true
scope of the invention is that defined in the following claims.
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